There is great interest in extracting economic benefit from the vast quantity of low and mid-grade heat rejected to the environment from primary energy generation and conversion sources. In the U.S., this amounts to approximately 59 quads of wasted thermal energy. Geothermal resources at temperatures between 100 and 200° C. represent an additional very large (100 GWth) and widely geographically dispersed resource. If even a fraction of the geo-pressured and co-produced resource fluids could be used to provide power, economically, it could easily quadruple the United States geothermal energy production output.
The principal reason why these thermal resources are not more fully exploited is cost. Low grade heat sources have modest exergy that can be turned into useful work. This means that standard power generation equipment, heat exchangers, expanders, pumps, etc., are significantly larger for the amount of power generated and efficiency of standard thermodynamic cycles such as the Organic Rankine Cycle (ORC) are inherently low, less than 10% typically. Moreover, conventional ORC power systems require bulk liquid condensation and vaporization to operate. These systems can fail to produce power at high ambient temperature and low driving temperature conditions because pressure drop across the expander eventually becomes too small to drive the engine and also achieve condensation of the working fluid in the condenser.
The resulting high capital and operating costs relative to the amount of revenue that can be generated from power sales is rarely very economically attractive, especially today with low costs for natural gas. Hence there is a need for new systems and methods that can dramatically bend the cost curve of power production from low grade heat sources.